The present invention relates to a process for metallization of metal substrates and relates more particularly to the hot metallization of pipes, in particular cast-iron pipes which are obtained by means of vertical continuous casting.
Among the known processes for the hot metallization of pipes, the following are known:
customized hot-galvanization; PA1 the continuous coating of steel plates with Zn or ZnAl; PA1 methods for thermal spraying with Zn or ZnAl alloys; PA1 zinc-coating of pipes. PA1 cooling the pipe in an atmosphere which is inert with respect to oxidation up to a temperature of 700 to 900.degree. C.; and PA1 spraying onto the pipe, cooled beforehand to the aforementioned temperature, a zinc-based metallization alloy with the aid of a set of spray guns arranged so as to surround the path of the pipe to be metallized.
The existing solutions based on galvanization have the following drawbacks.
Customized hot-galvanization coats the pipe internally and externally and the pipe undergoes an increase in temperature such that the results of other prior operations carried out on the pipe at lower temperatures risk being affected.
Hot galvanization by movement through a liquid poses the problem, currently unsolved, of tightness with respect to the liquid bath between the vertical pipe and the liquid metal, as well as the problem of maintaining a bath which is clean (without matte) and has a stable composition over time.
Cold metallization associated with annealing would ensure good distribution if rigorous conditions for preparation of the pipe surface are satisfied and would probably require heat treatment in an inert atmosphere.
The cost of such a treatment cycle would be considerable and as a result would offer limited advantages from an economic point of view.
The pipe to be coated, emerging after continuous casting, is not rough, which makes fixing of a cold coating impossible without carrying out surface preparation.
In order to obtain a well-distributed coating, a pipe without an oxide skin is necessary and the working temperature must be high or else the pipe handling time must be long.